Folic acid is already recommended to prevent birth defects, but the supplement could lower risk of autism as well.

In the U.S., grain and cereal makers have been supplementing breads and other products with folic acid since 1998, after studies showed that the B vitamin could lower the risk of neural tube defects in newborns. The latest study, published in the journal JAMA, provides another reason that moms-to-be should consume the recommended daily dose of 400 micrograms of folic acid: it may lower risk of autism in their newborns by up to 40%.

Researchers studied data from about 85,000 Norwegian children born between 2002 and 2008; in Norway, unlike in the U.S. there are no requirements to fortify foods with folic acid, so the researchers tracked the use of folic acid supplements in expectant moms beginning four weeks before they became pregnant to eight weeks into their pregnancy. Women who took the pills were less likely to have children diagnosed with autism spectrum disorders than mothers who didn’t take folate supplements. By the end of the study, when the children were 3 years to 10 years old, 270 were diagnosed with autism spectrum disorders (ASDs); 114 were diagnosed with autistic disorder, 56 with Asperger syndrome, and 100 with pervasive developmental disorder-not otherwise specified (PDD-NOS).

“Mothers who took folic acid supplements in early pregnancy had a 40% reduced risk of having children with autistic disorder compared with mothers who did not take folic acid,” says study author Dr. Pal Surén of the Norwegian Institute of Public Health. There was no drop in risk for developing PDD-NOS, and the number of children with Asperger syndrome was too low to be significant.

Folic acid is the synthetic form of the B vitamin folate, which occurs naturally in fruits and vegetables and is important for generating and maintaining healthy cells. That’s why it may prevent neural tube birth defects early in fetal development, and could explain why it also plays a role in neurodevelopmental disorders such as autism. In a previous study involving the same cohort of Norwegian babies, researchers found that children born to mothers taking folic acid supplements were less likely to show language delays by age three than those whose mothers did not take the supplements. The fact that the protective effect appears only during early pregnancy, further supports the idea that the nutrient may be preventing some of the molecular abnormalities that trigger autism, which seems to develop during the initial stages of fetal development.

“Folic acid appears to have a protective effect, and given that it only occurs when you take it early, that tells you something,” says study author Ian Lipkin, director of the Center for Infection & Immunity at Columbia University’s Mailman School of Public Health. “No one really knows precisely when autism comes about. The extent to which it comes early or later isn’t entirely clear either. This suggests the protective effect of folic acid for autism is generated early. Although we don’t understand the mechanisms, the recommendation is that people should start supplements when they want to get pregnant.”

The researchers acknowledge that they were not able to adjust for other factors that might account for the lower risk of autism among the mothers taking folic acid; women who take supplements, for example, are more likely to come from higher socioeconomic backgrounds and are able to take advantage of prenatal programs that could improve their health in other ways as well. But Surén and his colleagues also asked the mothers about their use of fish oil supplements, and did not find a similar drop of autism rates among these users, who presumably would be of a similar economic background as the women taking folic acid supplements.

The findings certainly raise an intriguing, and potentially simple way to lowering risk of autism, especially given the rising rates of the disorder in many parts of the world. “We have an explosion of autism worldwide,” says Lipkin. “Overall, the rate appears to be much higher than we anticipated. Some people argue the change is from a real increase while others argue it’s from more diagnosis. We still want to prevent it. Given that this is such a simple thing to do, it is inexpensive, non toxic, it is an important thing to tell anyone considering getting pregnant to take folate.”